Process And Apparatus For Depositing Semiconductor Layers Using Two Process Gases, One Of Which is Preconditioned

ABSTRACT

A method and device for depositing at least one layer, particularly a semiconductor layer, onto at least one substrate, which is situated inside a process chamber of a reactor while being supported by a substrate holder, is provided. The layer includes of at least two material components provided in a fixed stoichiometric ratio, which are each introduced into the reactor in the form of a first and a second reaction gas, and a portion of the decomposition products form the layer, whereby the supply of the first reaction gas, which has a low thermal activation energy, determines the growth rate of the layer, and the second reaction gas, which has a high thermal activation energy, is supplied in excess and is preconditioned, in particular, by an independent supply of energy.

PRIOR APPLICATIONS

This application is a divisional patent application of U.S. patentapplication Ser. No. 11/262,874 for a “Process And Apparatus ForDispositing Semiconductor Layers” filed on Oct. 31, 2005, which is acontinuation of International Patent Application No. PCT/EP2004/002994filed on Mar. 22, 2004, which designates the United States and claimspriority of German Patent Application No. 10320597.7 filed on Apr. 30,2003.

FIELD OF THE INVENTION

The invention relates to a process for depositing at least one layer, inparticular semiconductor layer, on at least one substrate carried by asubstrate holder in a process chamber of a reactor, the layer consistingof at least two material components which are in a controlled (fixed orvarying) stochiometric ratio and are respectively introduced in the formof a first reaction gas and a second reaction gas into the reactor,where the reaction gases are chemically decomposed as a result of asupply of energy, and some of the decomposition products form the layer,the supply of the first reaction gas, which has a low thermal activationenergy, determining the growth rate of the layer, and the secondreaction gas, which has a high thermal activation energy, being suppliedin excess and in particular being preconditioned by additional supply ofenergy, the first reaction gas flowing in the direction of the substrateholder through a multiplicity of openings which are disposed distributedover a surface of a gas inlet member lying opposite the substrateholder.

Furthermore, the invention relates to an apparatus in particular forcarrying out the process, having a process chamber disposed in areactor, which process chamber has a substrate holder for at least onesubstrate, having a heating apparatus for heating the substrate holderto a process temperature, having a gas inlet member, which lies oppositethe substrate holder, for introducing a first reaction gas into theprocess chamber, the gas inlet member having a multiplicity of openingsfor discharging the first reaction gas, which openings are disposeddistributed over the surface of the gas inlet member lying opposite thesubstrate holder, and having an apparatus for preconditioning a secondprocess gas which is to be introduced into the process chamber.

BACKGROUND OF THE INVENTION

CVD systems, and in particular MOCVD systems, are used to producelight-emitting diodes, in particular green, blue and also whitelight-emitting diodes. Compound nitrides are deposited on asemiconductor surface in order to deposit white light-emitting diodes.In the production processes used hitherto, first and second processgases, for example in the form of TMG or NH₃, have been passed into theprocess chamber, where the process gases decompose or react with oneanother, and the reaction or decomposition products are deposited on thesurface of a substrate so as to form a layer or layers. The processesused hitherto are expensive, since the outlay on materials, inparticular with regard to the nitrogen hydride, is considerably higherthan the outlay on materials for the metal alkyl, for example TMG. NH₃,PH₃ or ASH₃ have to be introduced into the process chamber in aconcentration which is higher by an order of magnitude than the metalalkyls. Although the hydrides are relatively inexpensive compared to thealkyls, the consumption costs are approximately the same, on account ofthe high consumption. The high consumption is a result of the highthermal activation energy of the hydrides compared to the activationenergies of the metal alkyls.

To promote decomposition of the reaction gases, U.S. Pat. No. 4,539,068proposes that a plasma be ignited between the gas inlet member and thesubstrate holder.

U.S. Pat. No. 3,757,733 also proposes a plasma in the process chamberfor this purpose.

U.S. Pat. No. 6,289,842 B1 deals with the deposition of semiconductorlayers in the MOCVD system, in which the process gases are introducedinto the process chamber through a showerhead.

A plasma pre-treatment of a reaction gas is also known from JP08-167596.

WO 01/46498 describes introducing the alkyls separately from thehydrides.

SUMMARY OF THE INVENTION

Working on the basis of the situation which has been outlined above,whereby the light-emitting diodes produced using the known processes arenot in widespread use for cost reasons, the invention is based on theobject of providing measures which allow the lumen/cost ratio to beconsiderably enhanced.

The light yield with respect to the production costs incurred isimproved, according to the invention, by virtue of the fact that onlythe second process gas, which is introduced into the process chamberseparately from the first process gas, is preconditioned before itenters the process chamber. The decomposition products enter the processchamber at the edge of the substrate holder, immediately above it, andwithin the diffusion boundary layer diffuse parallel to the substrateholder surface. The apparatus which is proposed according to theinvention for carrying out this process is distinguished by the factthat a preconditioning apparatus is disposed at the edge of thesubstrate holder for preconditioning purposes. The substrate holder ispreferably in the shape of a ring, and the ring can rotate about itscenter. The preconditioning apparatus is then located in the center ofthis ring (at the ring inner edge). However, there is also provision forthe process chamber to be linear in form or in the shape of a funnel. Inthis case, the substrate holder is preferably in the shape of arectangle or a trapezoid. The preconditioning device is then locatedupstream of the susceptor. The first process gas (metal alkyl), which ispreferably trimethylgallium, is introduced into the process chamberthrough a multiplicity of openings. In this case, the openings arelocated in the wall which lies directly opposite the substrate holder.The direction in which the gas flows in extends transversely withrespect to the surface of the substrate holder. The direction in whichthe gas flows out extends transversely with respect to the direction inwhich the gas flows in and parallel to the substrate surface, i.e.parallel to the wall. This wall forms a gas inlet member in showerheadform. Further openings, through which a carrier gas, for examplehydrogen or nitrogen, flows into the process chamber, are disposed inthe top of the process chamber, upstream and/or downstream of the gasinlet member based on the direction in which the gas flows out, which isoriented parallel to the surface of the substrate holder. The flow ofthis carrier gas is matched to the flow of the carrier gas which flowsin through the openings in the gas inlet member, in such a manner as toform a diffusion/flow boundary layer which is as flat as possible abovethe substrate holder. In this case, the flow/diffusion boundary layer isas far as possible in the lower half of the process chamber. Thepreconditioned second process gas is injected into the process chamberin the form of radicals within this diffusion/flow boundary layer. Toproduce the radicals, the preconditioning apparatus preferably has aplasma generator or a hot-wire apparatus or catalytic device or acombination of the above. This is used to heat the second process gas totemperatures which are such that it decomposes to a high degree.

The openings in the gas inlet member for the first process gas are soclose together that the gas jets which emerge from the openings do notstrike the substrate holder as individual gas jets, but rather press thesecond reaction gas, which enters transversely with respect to thedirection of these gas jets, flat onto the substrate holder. This allowsthe amount of material deployed for the second reaction gas to beconsiderably reduced. Whereas the concentration profile of the firstprocess gas with the low thermal activation energy is substantially flatand constant over the entire length of the substrate holder, theconcentration profile of the radicals decreases in the direction of themain gas flow direction. However, in this context it is ensured that theconcentration of the radicals is always greater than the concentrationof the first process gas immediately above the substrate. On account ofthis setting, the decomposition products of the second process gas arealways present in excess. The growth rate is determined by the supply ofthe first process gas. The excess supply of the decomposed secondprocess gas means that the layer which is deposited has a low number ofEPD defects. The concentration of defects is preferably below 10¹¹ cm⁻².On account of this lack of tendency to incorporate defects, it ispossible to achieve growth rates which are higher than those of theprior art and in particular also higher than 5 μm/h. According to theinvention, the second reaction gas may be a hydride. In particulararsine, phosphine, ammonia or UDMH are suitable. These gases can bealmost completely decomposed into radicals by thermal and/or catalyticmeans in the preconditioning apparatus. Therefore, the mass flow of thesecond reaction gas introduced into the preconditioning apparatus needonly be slightly greater than the mass flow of the first process gasintroduced into the gas inlet member. The mass flow of the first processgas, for example TMG, is typically a few sccm, for example 3 sccm. Themass flow of the hydride gas introduced into the preconditioningapparatus is only approximately three times this level.

In addition, it is also possible for a considerably greater mass flow ofa carrier gas to be introduced into the gas inlet member. The mass flowof the nitrogen or hydrogen used for this purpose may amount toapproximately 30 slm. On account of the virtually complete decompositionof the second process gas within the preconditioning apparatus, thesupply of decomposition products of the second process gas in the gasphase immediately above the substrate surface is nevertheless greaterthan the supply of the decomposed or undecomposed first process gas,which in addition to TMG may also be TMI or other metal alkyls. Theprocess temperatures can be varied within a wide range. They may bebetween 400° C. and 1600° C. The adverse effect on the temperatureprofile within the process chamber caused by a thermally preconditionedsecond reaction gas is negligible, on account of the relatively low massflow and heat capacity. It is important that the diffusion of thepreconditioned hydrides be directed transversely with respect to thealkyl gas stream emerging from a CCS showerhead. The carrier gas whichemerges from the showerhead together with the alkyl gas hydrodynamicallycompresses the flow of the preconditioned hydrides onto the crystalgrowth surface. The high quantity of carrier gas stream substanceresulting from it being fed in via the gas inlet member leads to such ahigh dilution of the hydrides at the location of the surface of the gasinlet member that the reaction equilibrium for the formation ofparasitic deposits on the gas inlet member is well below 1. The resultof this is that there can be longer intervals between cleaning of theprocess chamber than the intervals required in the prior art. On accountof the proposal according to the invention, the mass flow of the hydrideis reduced by a factor of 100 compared to the prior art. At the sametime, this reduces the defect density in the deposited layers, so thatlight-emitting diodes (GaN) which emit in the UV, produced in this way,can be operated with a higher current, i.e. with a higher light yield.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the apparatus according to the invention areexplained below on the basis of appended drawings, in which:

FIG. 1 shows in diagrammatic representation a tunnel reactor with afunnel-shaped process chamber;

FIG. 2 shows the reactor according to FIG. 1 in plan view onto thesubstrate holder;

FIG. 3 shows an alternative ring reactor in radial section;

FIG. 4 shows an alternative ring reactor in cross section;

FIG. 5 diagrammatically depicts the concentration of radicalsimmediately above the substrate surface in the direction of the main gasflow direction;

FIG. 6 shows an alternative form of reactor, illustrated in the sameview as in FIG. 2;

FIG. 7 shows an alternative form of reactor to the reactor according toFIG. 3;

FIG. 8 shows a further alternative for a reactor; and

FIG. 9 shows a further alternative in a highly diagrammaticrepresentation.

DETAILED DESCRIPTION OF THE INVENTION

The reactor 1 illustrated in FIG. 1 has a housing (not shown). Withinthe housing of the reactor 1 is a heating device 13 which can be used toheat a substrate holder 4 to process temperature. A substrate, on whicha layer is to be deposited, is located on the substrate holder 4. It isalso possible for there to be a multiplicity of substrates 5 on thesubstrate holder 4.

The process chamber 2 is located above the substrate holder 4. Theprocess chamber 2 is delimited at the top by a gas inlet member 3. Thisgas inlet member 3 forms a gas exit surface 18, which extends parallelto the surface 20 of the substrate holder 4. Gas entry openings 6 arelocated in the gas exit surface 18 in known manner. These gas entryopenings 6 are distributed over the gas exit surface 18 in such a mannerthat the gas jets which emerge from the gas entry openings 6 and enterthe process chamber in the gas inflow direction form a uniform gas flowfield in the direction of the substrate holder 4 in a region which islocated below the center of the process chamber height. However, anoutgoing flow of gas 16, which is oriented transversely with respect tothe gas inflow direction 11, is established above the flow/diffusionboundary layer 12.

Further gas entry openings are located upstream of the gas entryopenings 6. There are also further gas entry openings 8 downstream ofthe gas entry openings 6. Whereas not only a carrier gas in the form ofN₂ or H₂, but in particular also the first process gas, in the form ofTMG (trimethylgallium) or TMI (trimethylindium) can pass through theopenings 6 of the gas inlet member 3, only the carrier gases nitrogen orhydrogen enter the process chamber through the gas entry openings 7, 8adjacent to these gas entry openings 6, in order to condition theflow/diffusion boundary layer 12 in the edge region above the substrateholder 4 as well, running parallel to the surface 20.

In the exemplary embodiment illustrated in FIG. 6, the substrate holder4 has a rectangular surface when seen from above. In this case, theprocess chamber widens in the direction 16 in which the gas flows out.The process chamber has a cross section which remains constant over theentire substrate holder in the direction 16 in which the gas flows out.

The reactor illustrated in FIG. 3 also has the properties describedabove. Whereas the substrate holder 4 of the reactor illustrated inFIGS. 1 and 2 is in the shape of a trapezoid, the substrate holder 4 ofthe reactor illustrated in FIG. 3 is in the shape of a ring. A pluralityof substrates 5 are positioned on this ring-shaped substrate holder 4.The substrate holder 4 can be driven in rotation. The substrates lyingon the substrate holder 4 can likewise be driven in rotation in knownmanner. Heating of the substrate holder 4 is effected in known manner,either by means of RF heating or by thermal radiation.

It is important that only the alkyl together with a carrier gas flowsinto the process chamber 2 through the gas inlet member 3, which isconfigured in the form of a showerhead. The hydride, which may be NH₃,PH₃ or AsH₃, flows into a preconditioning apparatus 9 via a hydridefeedline 15. In the exemplary embodiment illustrated in FIGS. 1 and 2,the preconditioning apparatus 9 is located upstream of an edge 19 of thesubstrate holder, as seen in the main direction of flow 16. In theexemplary embodiment illustrated in FIG. 3, the preconditioningapparatus 9, only half of which is illustrated in that figure, islocated in the center of the annular interior space within thering-shaped substrate holder 4. In this case, the supply 15 of thehydride can be realized from below.

In the exemplary embodiment illustrated in FIG. 7, the substrate holder4 is likewise configured in the form of a ring.

In this case too, the injection of the radicals generated by thepreconditioning device 9 is effected from the edge 19 of the substrateholder. Unlike in the case of the exemplary embodiment illustrated inFIG. 3, however, the injection takes place from the outer edge, so thatcompressive diffusion with respect to the substrate surface is formed.This compensates for the depletion profile.

The preconditioning apparatus 9 is only symbolically illustrated in theschematic illustrations. The preconditioning apparatus 9 may be a plasmagenerator. However, it is preferable for the preconditioning apparatus 9to be a device for thermal decomposition of the hydride. This may be a“hot wire” device. The hydride is broken down into radicals by means ofthis device, which has wires that have been heated to high temperatures.The decomposition of the hydride into radicals is preferably virtuallycomplete.

The nitrogen radicals N⁺ produced in the exemplary embodiment fromammonia in the preconditioning apparatus 9 are passed into the processchamber 2 through an exit passage 10. The exit passage 10 forms aninjection opening and opens out into the process chamber, andspecifically into the diffusion boundary layer in the chamber,immediately above the surface 20 of the substrate holder 4. The exitpassage 10 in this case opens out at the edge of the substrate holder19. As a result, an N⁺ stream which extends parallel to the substratesurface 20 and is directed orthogonally with respect to the direction offlow of the gas jets 11 (inflow direction) is formed. The gas jets 11press the diffusion stream of the nitrogen radicals onto the surface ofthe substrate 5.

In the exemplary embodiment illustrated in FIG. 3, the apparatus forreceiving the preconditioning apparatus 9 is substantially cylindricalin form, with the cylinder being a covered hollow body. In the exemplaryembodiment illustrated in FIGS. 1 and 2, the exit passage 10 isconfigured in the shape of a funnel.

The alternative reactor 1 illustrated in FIG. 4 has a process chamber 2which is considerably higher than the process chambers of the exemplaryembodiments shown in FIG. 1 to 3. To compensate for this, the substrateholder 4, which is likewise ring-shaped, is driven at a higherrotational speed. This leads to the flow/diffusion boundary layer 12being “pulled flat” above the substrate surface 5. As a result, thedevice for receiving the preconditioning apparatus 9 does not need tohave a cover here. However, in this case too, depending on the processparameters it may be advantageous for there to be a cover. The radicalswhich emerge from the exit passage 10, which is directed upward, arediverted in the transverse direction with respect to the gas jets 11 andparallel to the surface 20 of the substrate holder by the flowimmediately above the exit passage 10.

FIG. 5 shows the concentration profile of the nitrogen radicals N⁺ whichemerge from the exit passage 10 in relation to the gallium concentrationin the gas phase above the substrate surface. The nitrogen radicals arepresent in excess, the concentration 17 of the nitrogen radicalsdecreasing in the direction of the main gas flow 16. However, the ratioN⁺/Ga remains >1 over the entire length (FIGS. 1 and 2) or radius (FIGS.3 and 4).

At typical process temperatures, which may be 400, 500, 600, 700, 800,900, 1000, 1100, 1200, 1300, 1400, 1500, −1600° C. and any temperaturein between or above, mass flows of trimethylgallium of between 2 and 10sccm are conducted into the process chamber 2. The mass flow of NH₃which is introduced into the preconditioning apparatus 9 through theline 15 is only at a low level, in particular is only slightly greaterthan the TMG mass flow, in particular only by a factor of 2 or 3. Bycontrast, the mass flow of the carrier gas (H₂, N₂) introduced throughthe openings 6, 7, 8 is greater by a factor of 1000 than the mass flowof one of the two reaction gases.

The invention can also be implemented using more than two reactiongases. In particular, there is provision for trimethylindium or TMAL orDcpMg to be introduced into the process chamber 2 in addition totrimethylgallium. It is also possible for other alkyls to be introducedthere. Furthermore, any other hydride, such as UDMH, can also beintroduced into the process chamber instead of or together with NH₃PH₃and/or AsH₃. These hydrides are preferably also preconditioned in themanner described above.

The hydrides can be introduced into the preconditioning apparatus 9 inconcentrated form or together with a carrier gas. It is preferable forthe introduction of the hydrides to be effected with very little carriergas, in order to minimize dilution of the preconditioned gas. Thetemperature in the preconditioning device may in this case be higher orlower than the process temperature in the process chamber.

In a variant of the invention which is not illustrated, there isprovision for the substrate holders themselves to be driven in rotation.They may in this case rest on a gas cushion driven in rotation. It ispreferable for the substrate holders, which are disposed in planetarymanner, to rest on individual substrate carriers, which are disposed inthe substrate holder 4 so as to be driven in rotation.

In the exemplary embodiment illustrated in FIG. 8, two different alkylsare introduced into the process chamber. In this case, each of the twoalkyls is conducted into a separate chamber 21, 22 of the gas inletmember 3. Each of the two chambers 21, 22 is provided with separate gasentry openings 6′, 6″, which open out into the process chamber. Thisprevents premature reactions between the individual metal alkyls.Reference is made to U.S. Pat. No. 5,871,586 for details of chambers ofthis type.

In the further exemplary embodiment illustrated in FIG. 9, the alkyl(s)is/are preconditioned in a special preconditioning device 23. In thiscase too, the hydrides are preconditioned in a preconditioning apparatus9. In this case, the supply of the hydrides 15 is effected from above.The preconditioning device 9 is located approximately at the height ofthe process chamber. However, in this case too, the injection of theradicals is effected through an exit passage 10 which is disposeddirectly at the edge of the substrate holder 4. The injection of theradicals is effected directly into the diffusion boundary layer.

This further preconditioning device 23 for the alkyls is located in theregion of the showerhead immediately in the region of the top of theprocess chamber. The preconditioned process gas, together with a carriergas, enters the process chamber through the gas entry openings 6 in themanner described above. The preconditioning apparatus 23 may be acooling device. The cooling may be effected by a cooling liquid or insome other way, for example by gas streams or by dissipation of heat.The heat can in this case be dissipated by way of an adjustable gas gap.

All features disclosed are (inherently) pertinent to the invention. Thedisclosure content of the associated/appended priority documents (copyof the prior application) is hereby incorporated in its entirety in thedisclosure of the application, partly with a view to incorporatingfeatures of these documents in claims of the present application.

1. An apparatus for carrying out a process of depositing at least onelayer on at least one substrate, said apparatus having a process chamberdisposed in a reactor, which process chamber has a substrate holder forthe at least one substrate, having a heating apparatus for heating thesubstrate holder to a process temperature, having a gas inlet member,which lies opposite the substrate holder, for introducing a firstreaction gas into the process chamber, the gas inlet member having amultiplicity of openings for discharging the first reaction gas, whichopenings are disposed distributed over the surface of the gas inletmember lying opposite the substrate holder, and having an apparatus forpreconditioning a second process gas which is to be introduced into theprocess chamber, characterized in that the preconditioning apparatus isdisposed at an edge of the substrate holder in such a manner that thesecond reaction gas flows parallel to the substrate holder surfaceimmediately above the substrate holder and transversely with respect toa direction in which the first process gas flows in.
 2. The apparatusaccording to claim 1, characterized in that the substrate holder isconfigured in the shape of a ring, and the preconditioning apparatus islocated in the space inside the ring.
 3. The apparatus according toclaim 1, characterized in that the substrate holder is in the shape of atrapezoid, and the preconditioning device is situated upstream of anarrow side of the trapezoid.
 4. The apparatus according to claim 1,characterized in that a cross-sectional area of the process chamberabove the substrate holder is constant in the direction in which the gasflows out.
 5. The apparatus according to claim 1, characterized in thatthe gas inlet member is configured as a closed capped showerhead.
 6. Theapparatus according to claim 1, characterized in that openings areadjacent to the openings in the gas inlet member both upstream anddownstream of them in the main direction of gas flow, through whichupstream and downstream openings carrier gas is introduced into theprocess chamber in a direction which is transverse to the main directionof gas flow, for the purpose of conditioning a diffusion/flow boundarylayer.
 7. The apparatus according to claim 1, characterized in that thepreconditioning apparatus includes a plasma generator.
 8. The apparatusaccording to claim 1, characterized in that the preconditioningapparatus has a heater.
 9. The apparatus according to claim 1,characterized in that a process chamber height is >75 mm and arotational speed of the substrate holder, which is driven in rotation,is >100 rpm.
 10. The apparatus according to claim 1, characterized inthat the injection of radicals takes place at an outer edge of aring-shaped substrate holder.
 11. The apparatus according to claim 1,characterized in that metal alkyls are supplied through separate gasentry openings via a gas inlet system which includes two chambers. 12.The apparatus according to claim 11, characterized by a preconditioningdevice, which is associated with the gas inlet member for the metalalkyls.
 13. The apparatus according to claim 5, characterized in thatthe showerhead has a process chamber height of between 10 mm and 75 mm.